No, you cannot use the visa approval letter to get the visa stamp at the border by land or sea. Vietnam visa on arrival is applicable to air travel only since the visa applicants must get visa stamped at the arrival airport in Vietnam. If this is the case, you are recommended to contact Vietnam Embassy/Consulate in your living country or near your living county to apply for a regular visa, or you can obtain an Evisa.
An Evisa is an official document permitting entry into and travel within Vietnam. This type of visa is an alternative to visas issued at Vietnam Immigration Dept and at the ports of entry. Please notice that an evisa to Vietnam once issued is valid for 30 days with single entry and for tourism purpose only. Please send your passport scan + 1 passport photo in white background for visa processing. In addition, you should inform us the exact arrival and exit port. It takes 3 working days for tourists to find out if their applications have been approved or not, according to the directive. The fee is $44US/pax and non refundable.